RAIN WATER COLLECTION & HARVESRING

Chapter 3

Components of Rainwater Harvesting System

3.1 Roof Catchment

In a rainwater harvesting system, the only area for house owner to harvest rainwater is the

roof. However, if the users want to increase additional capacity, an open barn or rain barn

can be built beside the house roof. Harvested rain water quality can varied according to

different type of roof catchment material, country’s climate, and surrounding

environment (Vasudevan, 2002).

3.1.1 Metal Roof

Smoother surface of roof can enhance the harvesting ability. A common used roofing

material for rainwater harvesting is Galvalume, which consists of 55% aluminum and

45% zinc alloy-coated sheet steel (Texas, 2005).

3.1.2 Clay or Concrete Roof

Clay and concrete tile are porous. These types of materials are suitable for potable system

as well as non-potable system. However, it may cause 10% of runoff loss due to the tiles

texture. The solution is to coat it with sealant to reduce loss. However, sealant may have

chance of toxins leaching even though it can prevent bacterial growth(Texas, 2005).

3.1.3 Roof Area Calculations

The size of roof area has a huge impact on the collection of rainwater for a house or

building. Before calculating the roof area, it is important to determine which parts of roof

can be used for collecting rainwater. Figure 3.1.1 shows three different types of roof

slope along with their formulae for roof area calculations(DID, 2012)

(a) Single Sloping Roof Freely Exposed to the Wind

Ac=Ah+Av

2Eq. 3.1a

(b) Single Sloping Roof Partially Exposed to the Wind

Ac=Ah+12( Av 2−Av 1) Eq. 3.1b

(c) Two Adjacent Sloping Roofs

Ac=Ah1+ Ah 2+12( Av2−Av 1) Eq. 3.1c

Figure 3.1.1: Roof Catchment Areas(DID, 2012)

3.2 Gutter and Downpipe

Gutters are used to capture the rainwater running off from the roof and downpipes are

used to deliver the rainwater into the rainwater storage tank.Inadequate number of

downpipes, excessive long roof length, steep roof slopes, and less perform gutter

maintenance, are among the reasons of spillage or overrunning of rainwater. Therefore, it

is advisable to consult the gutter supplier for the best installation.

In allocating potable usewater system, gutter and downpipes cannot use lead

material. This is due to slightly acidic quality of rain could dissolve lead and thus

contaminate the water supply.The most common materials ofgutters for both potable and

non-potable systemsare PVC,vinyl, seamless aluminum and galvanized steel(Georgia,

2009).Figure 3.2.1 shows the typical half round gutters and eaves gutters.

(a) Layout of half round (b) Eaves Guttersgutters& downpipes

Figure 3.2.1: Examples of Roof Conveyance System

3.2.1 Roof Catchment Runoff Calculation

The general equation used to calculate the rainwater runoff flow rate on the roof is as

below:

Q (l/s) = catchment area (m2) x rainfall intensity (mm /hr) x impermeability factor ÷ 3600

(Eq. 3.2a)

However, for roof slope greater than 40°, the following equation is adopted:

Q(l/s) = catchment area (m2) x rainfall intensity (mm /hr) x (1+ 0.462 tanϴ) x impermeability

factor÷ 3600

(Eq. 3.2b)

Whereϴ is the roof pitch in degrees.

3.2.2 Calculations ofGutter and Downpipe Sizes

Equation 3.2c is used to calculate the size of the level half-round gutter based on the

calculated Q by Equation 3.2a or 3.2b:

Q = 2.67 x 10−5 x Ag1.25 l/s (Eq. 3.2c)

Where Agis cross sectional area of the half-round gutter in mm2.

On the other hand, Equation 3.2d is used to calculate the size of other shapes of level

gutter based on the calculated Q by Equation 3.2a or 3.2b:

Q = 9.67

105 x √ Ao3

W l/s (Eq. 3.2d)

Where Aois the cross sectional area of flow at gutter outlet in mm2, and W is the width of

water surface.

Note:

For 1:600 gradient of gutter,Qis increased by 40%; while the frictional resistance of

gutter can reduce Q by 10% and each bending can reduce 25% of Q.

Tables 3.2.1a, 3.2.1b, 3.2.2a, 3.2.2b, 3.2.3a and 3.2.3bshowthe sizes of half

round and rectangular gutters with downpipes calculated based on Equations 3.2a-

3.2dfor various design rainfall intensities of50-mm/h, 100-mm/h and 150-mm/h. The

calculations were based on the assumptions of (i) roof pitch is 30o; (ii) 1:600 gradient of

gutter and Q is increased by 40%; (iii) the frictional resistance of gutter can reduce Q by

10%; and (iv) no bending gutter. User can select the gutter and downpipe sizes from the

tables according to the roof area (m2), roof catchment runoff rate (L/s), and the shapes of

gutter and downpipe.

Table 3.2.1a:Half round gutters and downpipes for50-mm/h of designrainfall intensity

RoofArea(m2)

Roof RunoffRate(L/s)

Half Round Gutters (diameter/mm)

Circular Downpipe * (diameter/mm)

End outlet Center Outlet End outlet Center OutletCal. Size

Ava. Size

Cal. Size

Ava. Size

Cal. Size

Ava. Size

Cal. Size

Ava. Size

50 0.66 85 174 42.5 174 56.0 82 28.0 8260 0.79 90 174 45.0 174 59.5 82 29.5 8270 0.92 95 174 47.5 174 63.0 82 31.5 8280 1.06 100 174 50.0 174 66.0 82 33.0 82100 1.32 110 174 55.0 174 72.5 82 36.5 82120 1.58 120 174 60.0 174 79.0 82 39.5 82150 1.98 130 174 65.0 174 86.0 110 43.0 82200 2.64 145 174 72.5 174 95.5 110 48.0 82

*Downpipe size is 66% of gutter width

Table 3.2.1b:Rectangulargutters and downpipes for50-mm/h of design rainfall intensity

RoofArea(m2)


Rectangular/ Eave Gutters(mm)

Rectangular Downpipe *(mm)

Cal. Size Ava. Size Cal. Size Ava. Sizewidth depth width depth width depth width depth

50 0.66 75 37.5 190 150 49.5 25.0 100 5060 0.79 80 40.0 190 150 53.0 26.0 100 5070 0.92 85 42.5 190 150 56.0 28.0 100 5080 1.06 90 45.0 190 150 59.5 30.0 100 50100 1.32 95 47.5 190 150 62.5 31.5 100 50120 1.58 105 52.5 190 150 69.5 35.0 100 50150 1.98 115 57.5 190 150 76.0 38.0 100 50200 2.64 125 62.5 190 150 82.5 41.0 100 50

*Downpipe size is 66% of gutter widthNote: Assumedthe depth is half of the width of gutter

Table 3.2.2a:Half round gutters and downpipes for100-mm/h of design rainfall intensity

RoofArea(m2)





Ava. Size

Cal. Size

Ava. Size

Cal. Size

Ava. Size

Cal. Size

Ava. Size

50 1.32 110 174 55.0 174 72.5 82 36.5 8260 1.58 120 174 60.0 174 79.0 82 39.5 8270 1.85 125 174 62.5 174 82.5 82 41.0 8280 2.11 135 174 67.5 174 89.0 110 44.5 82100 2.64 145 174 72.5 174 95.5 110 48.0 82120 3.17 155 174 77.5 174 102.5 110 51.0 82150 3.96 170 174 85.0 174 112.0 110 56.0 82200 5.28 195 174 97.5 174 128.5 160 64.5 82



RoofArea(m2)





50 1.32 95 47.5 190 150 62.5 32 100 5060 1.58 105 50.0 190 150 69.5 35 100 5070 1.85 110 105.0 190 150 72.5 36 100 5080 2.11 115 57.5 190 150 76.0 38 100 50100 2.64 125 62.5 190 150 82.5 41 100 50120 3.17 135 67.5 190 150 89.0 45 100 50150 3.96 150 75.0 190 150 99.0 50 100 50200 5.28 165 82.5 190 150 109.0 55 120 80


Table 3.2.3a:Half round gutters and downpipes for150-mm/h of design rainfall intensity

RoofArea(m2)





Ava. Size

Cal. Size

Ava. Size

Cal. Size

Ava. Size

Cal. Size

Ava. Size

50 1.98 130 174 65 174 85.8 110 42.9 8260 2.38 140 174 70 174 92.4 110 46.2 8270 2.77 150 174 75 174 99 110 49.5 8280 3.17 160 174 80 174 105.6 110 52.8 82100 3.96 170 174 85 174 112.2 110 56.1 82120 4.75 185 174 92.5 174 122.1 160 61.05 82150 5.94 200 174 100 174 132 160 66 82200 7.92 225 174 112.5 174 148.5 160 74.25 82



RoofArea(m2)





50 1.98 115 57.5 190 150 75.9 38 100 5060 2.38 120 60 190 150 79.2 40 100 5070 2.77 130 65 190 150 85.8 43 100 5080 3.17 135 67.5 190 150 89.1 45 100 50100 3.96 150 75 190 150 99 50 100 50120 4.75 160 80 190 150 105.6 53 120 80150 5.94 175 87.5 190 150 115.5 58 120 80200 7.92 195 97.5 250 178 128.7 64 150 75


Some local gutter manufacturers also produce gutters and downpipes with

different sizes from those stated in the tables above, such as 4” x 4” and 3½” x 6” eave

gutters.

Sample Calculation

Given:

For roof with 15-m of width and 4-m of length, the roof catchment area (ABCK) = 60-m2

Design rainfall = 100 mm/hRoof pitch ≈ 30°Roof permeability factor = 0.95

(i) Roof catchment runoff rate

Based onEquation 3.2a (less than 40 degree of roof pitch), the roof catchment runoff rate is:

Q = 60 m2 x 100 mm/hr x 1/3600 s/hr x 1/1000 m/mm x 1000/1 l/m x 0.95 = 1.58- l/s

Figure 3.2.2: Roof Catchment Area

(ii) Gutter size

Case I: Circular gutter (end outlet)

Cross sectional area of circular gutter, Ag, was calculated using Equation3.2c;Used1:600gradient of gutter, wheregutter flowrate increased by 40%;Fictional force reduced gutter flowrate by 10%Thus,

Q = 1.4 x 0.9 x 2.67 x 10−5 x Ag1.25 l/s

AndAg= 5472.07mm2

Width of gutter,

W = √(Ag x 8 / π)= 118.04-mm (rounded to 120-mm)

For center outlet, adopt a smaller gutter size (half of the size) as it only carries

half the flow load.

Case II: Rectangular gutter (end outlet)

Cross sectional area of rectangular gutter, Aowas calculated using Equation3.2d;Used1:600gradient of gutter, where gutter flowrate increased by 40%;Fictional force reduced gutter flow rate by 10%

Thus,

Q = 1.4 x 0.9 x 9.67

105 x √ Ao3

W l/s

AndAo=WD/2

Width of gutter (assumed that the depth is half of the width),

1.5833 = 1.4 x 0.9 x 9.67

105 x √ W 2 D3

8= 1.4 x 0.9 x

9.67

105 x √ W 5

64

W = 101.56-mm (rounded to 105-mm)

(iii) Downpipe size (for both circular and rectangular gutters)

Assumed to be 66% of gutter width, thus:

Case I:

Circular gutter should adopt 79-mm diameter of downpipe for end outlet design and 39.5-mm diameter of downpipe for center outlet design;

Case II: Rectangular gutter should adopt 69.5-mm widthand 35-mm depth of downpipe

3.2.2.1 MSMA’s Method for Eave Gutter Design

Table 3.2.4shows the sizes of eave gutters read from Charts 3.2.1 and 3.2.2from a

design rainfall intensity of 100-mm/h.Charts 3.2.1 and 3.2.2 show the relationships

among roof catchment area, design rainfall intensity and cross sectional area of

eavegutter. Finally,Table 3.2.5 shows the respective sizesof downpipes.

Table 3.2.4:Sizes of gutters and downpipes (DID, 2012)

RoofArea(m2)

Design RainfallIntensity(mm/h)

Cross Sectional Area of Eave Gutters (mm2)

Slope 1:500and steeper

Slope flatterthan 1:500

50 100 5400 720060 100 6250 825070 100 7000 940080 100 7800 1040090 100 8500 11250100 100 9250 12400

Chart 3.2.1:Eave Gutter Design Chart for Slope 1:500 and steeper (DID, 2012)

The chart assumes:

1) An effective width to depth is a ratio about 2:1:2) Gradient of 1:500 or steeper;3) Manning’s formula with ‘n’ = 0.0164) The least favorable positioning of downpipe and bends within the gutter length;5) Cross-section or half round, quad, ogee or square;6) The outlet to downpipe is located centrally in the sole of the eaves gutter.

Chart 3.2.2: Eave Gutter Design Chart for Slope flatter than 1:500 (DID, 2012)

The chart assumes:

1) An effective width to depth is a ratio about 2:1:2) Gradient of flow flatter than 1:500;3) Manning’s formula with ‘n’ = 0.0164) The least favorable positioning of downpipe and bends within the gutter length;5) Cross-section or half round, quad, ogee or square;6) The outlet to downpipe is located centrally in the sole of the eaves gutter.

Table 3.2.5: Required Minimal Nominal Size of Downpipe(DID, 2012)

Cross Sectional Area of Eave Gutters (mm2)

Minimal Nominal Size of Downpipe (mm)Circular Rectangular

4000

7565 x 50

42004600

75 x 504800

855900

100 x 506400

906600

75 x 706700

1008200

100 x 759600

12512,800 100 x 10016,000

150125 x 100

18,400150 x 100

19,200Not applicable20,000 125 x 125

22,000 150 x 125

3.2.2.2 Method from the Handbook of Rainwater Harvesting forCaribbean

Caribbean’s Rainwater Harvesting Handbook(UNEP, 2009)was intended as a practical

guideline to introduce and assist the citizens in the Caribbean region to construct their

rainwater harvesting systems. Thehandbook provides several technical information and

guidelines that are useful for tropical region like Malaysia. The rational method was used

to calculate the roof runoff.

Tables 3.2.6a and 3.2.6b show the recommended runoff coefficients for various

catchment types and the sizes of gutters and downpipes, respectively.Type of gutters

recommended here is PVC gutters since they do not rust and the rainwater quality can be

maintained over a long period. Gutters must slope towards the tank and the gradient must

not more than 1:100.

Table 3.2.6a: Runoff coefficients for various catchment types(UNEP, 2009)

Type of Catchment Runoff coefficientsRoof catchmentsTilesCorrugated metal sheets

0.8 – 0.90.7 – 0.9

Ground surface coveringsConcreteBrick pavement

0.6 – 0.80.5 – 0.6

Untreated ground catchmentsSoil on slopes less than 10 percentRocky natural catchments

0.1 – 0.30.2 – 0.5

Table 3.2.6b: Gutters and Downpipes sizing for RWH systems in tropical regions(SOPAC, 2004)

Roof area (m2)

served by one gutter

Gutter width (mm) Minimum diameter of downpipe (mm)

17 60 4025 70 5034 80 5046 90 6366 100 63128 125 75208 150 90

3.3 Type and Configuration of Rainwater Harvesting Systems

There are three types of rainwater harvesting system for supplying non-potable water for

internal and external uses, which are directly pumped, indirectly pump, and gravity fed

(Leggett et al.,2001).

3.3.1 Indirectly Pumped System

Indirectly pumped system is a system that pumps up water from storage tank to header

tank using water pump. The header tank is usually placed on the roof of building while

the storage tankis being undergrounded. When the storage tank runs out of water, the

primary water supply piping will supply water into header tank. When storage tank is full

with water, an overflow pipe is necessary to prevent storage tank from more than the

normal water level in a tank. The main advantage of indirectly pumped system is the

supply of water will not be cut-off if the water pump is on mechanical or electrical

failure. The water can still be supplied to internal and external uses by the primary water

supply system. The main disadvantage is the water can be delivered slowly due to low

pressure. Thus, it leads to low water pressure when using shower or slow refilling after

flushing the toilet. In addition, there may be insufficient space on the roof to install the

header tank.

Figure 3.3.1 : Indirectly Pumped System

3.3.2 Directly Pumped System

The main difference between indirectly and directly pumped system is the water in

directly pumped system is stored in storage tank and then pumped directly to internal and

external uses in a building. There is no header compared to indirectly pumped system and

the main water supply is direct from the storage tank. The main water supply will not

fully fill the tank but maintain the water on a minimum level for short term demand. The

main advantage of this system is the water is provided on high pressure. The

disadvantage is there will be no water supply if the water pump is experiencing faulty due

to mechanical or electrical failure.

Figure 3.3.2 :Directly Pumped System

3.3.3 Gravity Fed System

The main difference of gravity fed system from directly and indirectly pumped systems is

the storage tank of this system is located on top of the building. Rainwater which is

collected and harvested is directly stored in the storage tank or also known as header

tank. Water is delivered from storage tank by means of gravity to appliances. The main

advantage of the system is water pump or electrical supply to pump water is not required.

Since no pump is required, there is no risk of water pump failure and electrical supply

cut-off. However, the main disadvantage is the low water pressure similar in indirectly

pumped system. For example slow refilling in toilet tank after flushing the toilet.

Figure 3.3.3 :Gravity Fed System

3.3.4 Examples of Rainwater Harvesting System

3.3.4.1 Residential House –Bungalow, Semi Detached and Terrace

Figure 3.3.4: Schematic diagram forresidentialhouse

Basic components

Catchment surface, gutters, leaf guarder, downpipes, first flush diverters, storage tanks

(cisterns), overflow pipe

System Procedure:

1) Harvest rainwater from roof;

2) Filter roof dirt throughleaf guarder and first flush diverter;

3) Deliver rainwater using gutter and downpipe;

4) Storing clean rainwater in the storage tanks;

5) In case there is shortage of water or no rainfall, the public water supply is topped

up into the storage tanks.

3.3.4.2 High Rise Building –Condominium, Commercial and Office Tower

Figure 3.3.5: Schematic diagram forhigh-rise building

Basic components

Catchment surface, gutters, leaf guarder, downpipes, first flush diverters, storage tanks

(cisterns), pump, elevated rainwater header tank

System Procedure:

1) Harvest rainwater from roof;

2) Filter roof dirt through leaf guarder and first flush diverter;

3) Deliver rainwater using gutter and downpipe;

4) Collecting the first flush rainwater before water entering the storage tank.

5) Storing clean rainwater in the on the ground storage tank;

6) The pump is used to lift the rainwater up to the elevated rainwater header tank

installed on the roof top of the high rise building;

7) In case there is shortage of water or no rainfall, the public water supply is topped

up into the elevated rainwater header tank.

3.4 Pumping System

There are two types of pumping systems, namely pressurized water supply system and

header pressure system. For pressurized system, pressure tank is required to maintain the

pressure throughout the system. Pump is functioning when the pressure is drop. For

header pressure system, water is lifted up by the pump from storage tank to elevated tank,

and the water supplies to devicesby gravity force(UNEP, 2009).

3.4.1 Selection of Pump

To select an appropriate type of pump for a rainwater harvesting system, the following

five steps must be followed(Alberta, 2010):

StepI: Select the appropriate pump

Submersible pump and jet pump are the most common types of pump that used in the

residential house. Submersible pump is located inside the tank and can function properly

even fully submerged inside the water tank; while jet pump is located outside the tank.

Comparisonsamong submersible pump, jet pump and centrifugal pump, and their

advantages and disadvantages, are shown in Table 3.4.1.

Step II: Select the configuration of pump (speed of flow rate)

Two available pump controllers can be selected to configure the rainwater supply system.

(a) Constant speed pump:

Following a large drop in the system pressure, a constant speed pump will activate

and pump water at a fixed rate to replenish the volume of water stored in elevated

tank;

(b) Variable Speed Drive (VSD) / Variable Frequency Drive (VFD) pump:

VSD/VFD pumps can increase or decrease the speed of the pump impeller to

provide more or less water as needed by the pressure system.

Table 3.4.1: Advantages and disadvantages of different types of pump(Alberta, 2010)

Type of Pump Usage AdvantagesDisadvantages

Submersible pump Pumping process inside water

Can be used for water supply, drainage, slurry and sewage pumping

More efficient have a longer lifespan than jet pumps

Reduces the amount of equipment and space needed outside of the rainwater tank

Low noise

Pump must be physically extracted from tank to perform inspection, repair and/or replacement

May be more difficult to detect pump dry running (or any malfunction) as operation of pump may not be audible

Pumps generally designed for vertical installation, but must be installed horizontally as vertical installation reduces usable capacity of cistern (increases dead space volume)

Jet pump Use for well systems

Pump can be easily inspected, repaired and/or replaced

Generally less expensive than submersible pumps

More difficult to commission than submersible pumps, as they must be ‘primed’

Pump must be located in a temperature controlled space (indoors, pump house, etc.)

Pump operation may be noisy

Centrifugal Pump For domestic and light industrial applications

Quiet operation and compact design

Easy installation

Not suitable for large building like shopping mall

Unable to provide constant pressure

Table 3.4.2: Advantages and disadvantages of constant speed and VSD/VFD pumps(Alberta, 2010)

Pump Controller Configuration

Advantages Disadvantages

Constant speed pump

Generally less expensive than VSD/VFD pumps

Ideal for applications where minor variations in water pressure and flow rate are acceptable (i.e., refilling toilet tanks after flushing and operating a garden hose)

Pressure tanks can be quite large for applications requiring high flow rates

Flow rate and system pressure may spike when pump activates, and pressure may drop if water demands are too high

Variable Speed Drive (VSD) / Variable Frequency Drive (VFD) pump

Provide constant pressure to fixtures, regardless of demand

Use very small pressure tanks, or micro-pressure tank inside the pump or control unit

Often have built in low/high voltage shutoff and dry run protection

Smaller space requirements in the building

Lower electricity consumption than comparable constant speed pumps.

Use of smaller pressure tanks requires a greater number of ‘pump starts’ potentially increasing pump wear

More expensive than constant speed pump systems

StepIII: Pump Flow Rate

The amount of flow that must be generated by the pump depends on the types and

number of fixtures connected to the distribution system. This means that at here, we must

consider the peak hour flow rate for that particular building before selecting the suitable

pumping system.

StepIV: Pump Head

Determine pump head is an important step especially for high rise building. The pump

pressure not only takes account on lifting up water supply but also along with friction

loss, and various type of minor loss.

StepV: Acceptability of the service

Last issue to consider is that whether usage of pump is acceptable for that building. Some

of the system may interrupted by pump downtime. For small residential housing, several

times of service interruption is acceptable. For multi-residential or commercial buildings,

it is important to avoid pumping service went down at peak hour. The most common way

is to install an elevated tank on top of the building.

3.4.2 Alberta’sMethod

Alberta (2010)provides simple way to calculate the required pump capacity. For

estimation of maximum peak demand flow rate, Table 3.4.3 can be used.Table 3.4.4

shows the required system pressurefor different indoor and outdoor fixtures.

Table 3.4.3: Minimum recommended water flow rate for various indoor & outdoorfixtures(Alberta, 2010)

Indoor Fixtures

Minimum Flow Rate(Per Fixture)

Outdoor Fixtures

Maximum Flow Rate(Per Fixture)

Shower or Bathtub19 LPM[5 GPM]

Garden hose with 13mm[1/2 in.] supply

11LPM[3GPM]

Lavatory1 LPM

[0.3 GPM]

Garden hose with 18mm[3/4 in.] supply

19LPM[6GPM]

Toilet2.7 LPM

[0.7 GPM]Irrigation system

Varies(Consult supplier/

contractor)

Kitchen Sink1.6 LPM

[0.4 GPM]

Washing Machine19 LPM[5 GPM]

Dishwasher7.6 LPM[2 GPM]

Table 3.4.4: Required minimum pressure for residential home fixture (Georgia, 2009)

Usage Pressure Pressure Flow ft m psi kPa GPM LPM

Impact Sprinkler 93 28 40 275.8 4.5 17.0Pressure washer 46 14 20 137.9 4.0 15.1Toilet 46 14 20 137.9 6.0 22.7Garden hose nozzle 81 25 35 241.32 5.0 18.9

Figure 3.4.1 shows the illustration of different kinds of pump heads such as static

lift, static height and friction loss in a pumping system.

Figure 3.4.1:Illustration for components of pump head

Thepump head can be calculated using following equations:

Pump Head (m, or ft) = Required System Pressure + Total Dynamic Head Eq. 3.4a

Where the required system pressureis the operating pressure required for rainwater

fixtures (275-415 kPa [~40 – 60 psi] for typical residential applications). If the final

discharge of a pumping system is into a rainwater header tank, then there will be no

required system pressure or equals to zero.

Total Dynamic Head = Static Lift + Static Height + Friction Loss Eq. 3.4b

In order to calculate the total dynamic head, the friction head loss must first be

calculated. Friction Loss formula is shown as below:

Friction Loss = [(LP−SE+ LF−SE) x F100−SE

100 m pipe] + [(LP−SU + LF−SU) x

F100−SU

100 m pipe]

Where,

Friction Loss = Combined Friction losses (m) for the service piping (SE) and

supply piping (SU)

LP = Linear length of pipe (m)

LF = Equivalent length of pipe fittings (m)

F100 = Friction loss per 100m of pipe

There are two distinct sections of rainwater pressure piping:

1) Rainwater serviced pipe:The section of pipe from storage tank to a jet pump (or

pressure tank/control unit for submersible pumps)

2) Rainwater supplied pipe:The section of pipe from jet pump (or pressure

tank/control unit for submersible pumps) to permitted fixtures

Table 3.4.5shows the value of friction head losses (m) based on the selected pipe

diameters and pipe flow rates, and Table 3.4.6 shows the equivalent length of pipe for

different fittings.

Table 3.4.5:Friction head losses for SCH40 PCV pipe at various flow rates(Alberta, 2010)

Flow Rate, Q(LPM)

F100 Friction Head (m / 100m pipe)Pipe Diameter

13mm[1/2 in.]

18mm[3/4 in.]

25mm[1 in.]

32mm[1 ¼ in.]

38mm[1 ½ in.]

50mm[2 in.]

8 4.8 1.2 0.38 0.119 25.8 6.3 1.9 0.5 0.230 63.7 15.2 4.6 1.2 0.6 0.238 97.5 26 6.9 1.8 0.8 0.357 49.7 14.6 3.8 1.7 0.576 86.9 25.1 6.4 2.9 0.9113 13.6 6.3 1.8

Theabove table assumed a SCH40 PVC pipe or similar material such as PE-polyethylene

or PP-polypropylene is utilized.

Table 3.4.6:Equivalent length of pipe for different fittings(Alberta, 2010)

Fitting Equivalent Length of Pipe (m)Pipe Diameter

13mm[1/2 in.]

18mm[3/4 in.]

25mm[1 in.]

32mm[1 ¼ in.]

38mm[1 ½ in.]

50mm[2 in.]

75mm[3 in.]

90° Elbow 0.5 0.6 0.8 1.1 1.3 1.7 2.445° Elbow 0.2 0.3 0.4 0.5 0.6 0.8 1.2Gate Valve

(shut-off valve) (Open)

0.1 0.2 0.2 0.2 0.3 0.4 0.5

Tee Flow – Run 0.3 0.6 0.6 0.9 0.9 1.2 1.8Tee Flow –

Branch1.0 1.4 1.7 2.3 2.7 3.7 5.2

In Line Check Valve (Spring) or Foot Valve

1.2 1.8 2.4 3.7 4.3 5.8 9.8

Sample Calculation

A pumping system is installed at the bottom of a 2-storey house to elevate the water

supply from ground level to the elevated tank, as shown in Figure 3.4.2 below. It is

assumed that there are 3 bathrooms or WCs, and two valves and three 90° bending from

the harvested rainwater tank to the elevated rainwater header tank. Assumed that PVC

pipe is used.

Figure 3.4.2: Typical pumping system for a 2-storey house [Change static height to 8-m; draw and label the 2 valves; label 2 more bendings]

From Table 3.4.3:

The total toilet flow rate = 3 x 2.7-LPM = 8-LPM

From Table 3.4.5:

For a flow rate of 8-LPMwith rainwater serviced pipe and rainwater supplied pipe sizes

of 25mm [1 inch],

F100−SE = 0.38 m /100 m pipe

F100−SU = 0.38 m /100 m pipe

From Table 3.4.6:

For one (1) 90° bending in rainwater serviced pipe and two (2) 90° bending in rainwater

supplied pipe,

Serviced pipe with one 90° bending, LF−SE = 0.8 m

Supplied pipe with two 90° bending, LF−SU = 0.8 x 2 =1.6m

And,

Serviced pipe gate valve, LF−SE=¿ 0.2m

Supplied pipe gate valve, LF−SU=¿ 0.2m

Thus,

Friction Loss = [(LP−SE+ LF−SE) x F100−SE

100 m pipe] + [(LP−SU + LF−SU) x

F100−SU

100 m pipe]

= [(2+(0.8+0.2)) x 0.38

100 m pipe] + [(9+(1.6+0.2)) x

0.38100 m pipe

]

= 0.052-m

Then,

Total Dynamic Head = Static Lift + Static Height + Friction Loss

= 2 + 8 + 0.05244

= 10.052-m

3.4.3 Pump head

The required pump heads (in kPa and horsepower) for different flow rates and pipe sizes

calculated for a typical rainwater harvesting system, as shown in Figure 3.4.2, are shown

in Tables 3.4.7, 3.4.8 and 3.4.9.

Table 3.4.7: The required pump heads for 3/4-inch pipe size

Min Flow Rate, L/m

Static Lift, m

(A)

Static Height,

m

(B)

Friction Loss,

m

(C)

Total Dynamic

Head, m

(A+B+C)

* Cal. Pump Head, kPa(D)

#Req. Pump Head,kPa

(D) / 0.7

* Cal. Pump horsepower

(E)

# Req.Pumphorsepower

(E) / 0.78 2 8 0.158 10.158 100 142 0.014 0.009819 2 8 0.832 10.832 106 152 0.037 0.025930 2 8 2.006 12.006 118 168 0.066 0.046238 2 8 4.264 14.264 140 200 0.102 0.071457 2 8 6.560 16.560 162 232 0.181 0.126776 2 8 11.471 21.471 211 301 0.324 0.2268

* Direct discharge to rainwater header tank# Assumed 70% of pump efficiency

Table 3.4.8: The required pump heads for 1-inch pipe size

Min Flow Rate, L/m

Static Lift, m

(A)

Static Height,

m

(B)

Friction Loss,

m

(C)

Total Dynamic

Head, m

(A+B+C)


#Req. Pump Head,kPa

(D) / 0.7


(E)


(E) / 0.78 2 8 0.052 10.052 99 141 0.01 0.00719 2 8 0.262 10.262 101 144 0.03 0.02130 2 8 0.635 10.635 104 149 0.04 0.02838 2 8 0.952 10.952 107 153 0.06 0.04257 2 8 2.015 12.015 118 168 0.09 0.06376 2 8 3.464 13.464 132 189 0.14 0.098

* Direct discharge to rainwater header tank

# Assumed 70% of pump efficiency

Table 3.4.9: The required pump heads for 1 1/4 -inch pipe size

Min Flow Rate, L/m

Static Lift, m

(A)

Static Height,

m

(B)

Friction Loss,

m

(C)

Total Dynamic

Head, m

(A+B+C)


#Req. Pump Head,kPa

(D) / 0.7


(E)


(E) / 0.78 2 8 0.015 10.015 98 140 0.01 0.00719 2 8 0.074 10.074 99 141 0.03 0.02130 2 8 0.176 10.176 100 143 0.05 0.03538 2 8 0.380 10.380 102 145 0.07 0.04957 2 8 0.559 10.559 104 148 0.11 0.07776 2 8 0.941 10.941 107 153 0.15 0.105113 2 8 2.000 12.000 118 168 0.25 0.175

* Direct discharge to rainwater header tank# Assumed 70% of pump efficiency

3.4.4Loading Unit Method

Flow rate can also be estimated using the loading unit method. Table 3.4.10 shows the

loading unit rating for different types of appliances. After calculating the total loading

unit for a rainwater supply system, the rainwater flow rate can be read from Chart 3.4.1.

Table 3.4.10:Loading Unit Rating for Various Applications(DID, 2012)

Chart 3.4.1: Design Flow Rate (L/s) versus Loading Units (DID, 2012)

Sample Calculation

For a 10-storey tower, it is assumed that every floor consists of 10 units and each of the

unit has 2 bathrooms or WCs. The design flow rate is equal to:

From Table 3.4.10, the loading unit for W.C Flushing Cistern is 2 units.

Type of Appliance Loading Unit RatingDwelling andFlatsW.C. Flushing Cistern Wash Basin Bath Sink

21.5103-5

Offices W.C Flushing Cistern Wash Basin (Distributed Use) Wash Basin (Concentrated Use)

21.53

School and Industrial Buildings W.C Flushing Cistern Wash Basin Shower (with Nozzle)

233

Public Bath 22

Total loading units = Loading Unit for 1 unit x No. of units per floor x Total floor

= (2 x 2) x 10 x 10

= 400-units

From Chart 3.4.1:

Q = 3.51-l/s = 0.00351- m3/s

3.5 Top-up System

There is always a time when there is insufficient of rainwater to meet the demand. In this

situation, it is necessary to have another alternative water supply for the water supply

system. Top-up devicecan be used to solve this problem. When the water level inside the

rainwater tank is getting lower, the top up system will start filling up the rainwater tank

by transferring water from the public water supply.

Rainwater must notflow into the public water supply system. Water from the

public water supply can flow into the rainwater tank subjected to it being equipped with a

one-way non return valve, or the overflow pipe in the rainwater tank is located at least

225-mm lower from the inlet pipe to the rainwater tank(Selangor, 2012).

3.5.1 Types of Top-up System

There are various types of top-up system available for the rainwater supply system. It is

advisable to select the top-up system wisely to avoid overflow, storage run down or extra

expense for unnecessary system. Basically, there are two types of top-up systems namely

automatic top-up system and manual top-up system. Table 3.5.1 shows the advantages

and disadvantages of automatic top-up system and manual top-up system.

Table 3.5.1:Advantages and disadvantages of top-up systems (Canada, 2012)

Make-up water method

Advantages Disadvantages

Manual top-up

Simplest method to design and install due to reduced control equipment requirements

Lowest cost alternative

May result in service interruptions (for example, no water for flushing toilets) if tank not topped up prior to going dry

Requires homeowner to monitor volume of stored rainwater in tank and top up pre-emptively if low

Automatic top-up

Reduces the number of service interruptions by automatically filling tank before it runs dry

Make-up system operates without the need for monitoring or intervention by the homeowner

Improper design or installation of control equipment may cause insufficient or excessive top-up volumes to be dispensed by the make-up system

Service interruption during power failure

3.5.2 Automatic Top-up System (with electronic device)

There are two types of automatic top-up system, with electronic device and without

electronic device. The planning stages for the layout of the automatic top-up system (with

electronic device) are:

i. As shown in Figure 3.5.1, a top-up system consists of the following components:

(a) Water level sensors located in the rainwater storage tank;

(b) A solenoid valve located in the rainwater storage tank;

(c) An air gap;

(d) Top-up drainage conveying make-up water to the rainwater storage tank;

(e) Electrical conduits containing wiring from water level sensors and

pumps.

Figure 3.5.1: Schematic diagram of top-up system for rainwater supply system (Alberta, 2010)

ii. Determine the location of solenoid valve and air gap;

iii. Plan route of top-up drainage;

iv. Plan route of electrical solenoid valve and power supply to the tank;

v. Contact municipality and other service providers to ensure that the planning

layout do not conflict with the current building systems like sewerage, piping,

electricity, building structures, etc.

3.5.2.1 Control Equipment

The control equipment is shown in Table 3.5.2 below:

Table 3.5.2:Control equipment(Canada, 2012)

Control equipment

Description Devices/options available

Water level sensor

A device inside the tank is used to sense water level

Can control (turn on or off) warning lights, solenoid valves and/or pumps, based on water level

Float switch Ultrasonic level sensor Liquid levels switch (Float

switch is typically used for residential applications).

Shut-off valve A device that is manually opened (or closed) to permit (or prevent) the flow of water

Integrated into the RWH pressure system to manage water flow and isolate components of the makeup system (for example, solenoid valves and backflow preventers)

Types: ball valves, gate valves

Shut-off valves selected must be approved for handling water under pressure.

Solenoid valve (automated shut-off valve)

A valve that activates (opens or closes) automatically when turned on

Connected to water level sensor to activate make-up water system

Come in a variety of configurations

The solenoid valves selected must be approved for handling water under pressure

3.5.2.2 Installation Procedures

The installation procedures are as follow:

Step 1: Set suitable type of water level sensors for RWH system

Float switches:

Float switches is installed in the water tank so that it can be used to sense the water

level, then controlling the pump and top-up water. Generally, normally close (N/C)

float switch is used for top-up systems, and normally open (N/O) float switch is

used for pumping system.

*(N/O) float switch: Supply power (turn on) when switch is “down”

*(N/C) float switch: Supply power (turn off) when switch is in

“up”

Other water level sensors should be selected properly and installation procedure should

be handled carefully with applicable codes & regulations.

Step 2: Installation of solenoid valves and shut-off valves

(a) Select suitable valves type

(b) Valve size must be the same size as piping system.

(c) Top-up systems use an (N/C) solenoid valve.

(d) Solenoid valve should be installed on the top of the air gap and soft close/slow

close solenoid is recommended.

(e) Solenoid valves must be wired into a power supply in conjunction with water

level sensor.

(f) Must be installed by a licensed plumber or technician.

(g) If soft/slow close valve is not used, a water hammer arrester shall be installed

on public water supply piping upstream.

(h) All procedure should handle with care with applicable codes & regulations.

Step 3: Installation of Air Gap

Air gap is required to prevent backflow. It needs to be noted that rainwater cannot be

mixed with the public water supply)

(a) The air gap height must be at least 25-mm, 1-inch or twice the diameter of

water pipe.

(b) To prevent splash and water damage:

i. install flow restrictor at upstream of valve

ii. install aerator at place public water supply terminate

iii. extend length of pipe and cut an angle no less than 45° at end pipe

(c) All procedure should handle with care with applicable codes & regulations

3.5.3Automatic Top-up System (without electronic device)

Top-up valve, as shown in Figure 3.5.2, can effectively maintain supply when demand

exceeds the rainwater supply and it does not require electricity supply or complex float

switch devices.

3.5.3.1 OperatingPrinciple

Under normal conditions, rainwater will fill the tank. If the rainwater level drops below a

pre-set level, the top-up system valve will open to maintain the water level using the

mains water supply.

3.5.3.2 Installation Procedures

i. Valve must be installed horizontally;

ii. Do NOT install on an angle;

iii. Do NOT restrict inlet water flow;

iv. NOT to be modified;

v. NOT to be used in dual purpose tanks used for stormwater detention.

Figure 3.5.2: Schematic Diagram of Top-up Valve

3.6 Leaf Guarder

Leaf guarder, which is also known as leaf screen or gutter guarder, fit along the length of

the gutter. It is one of the filter components at the pre-treatment stage of the rainwater

harvesting system. It is usually a ¼-inch mesh screens in wire frames. They do come in

aluminium, plastics and vinyl for user’s requirement.

Purpose of installing leaf guarder is to separate the leaves and other debris those

are washed down from the roof catchment surface. The leaf guarder is said to be the first

stage filtration that screen out the large particles such as leaves, bloom and twigs in the

collected rainwater. Through removing the large particles, the subsequent components

and devices in the rainwater harvesting system are said to be protected as accumulation of

the large particles in the system may deteriorate the quality of the rainwater. Simple

maintenance is required to clean the leaf guarder regularly. Decomposition of the leaves

and other debris may expand the bacteria activities and cause harmful consequences to

the rainwater collected. Maintenance may be operated weekly or monthly depend on the

debris accumulation speed.

In Malaysia, according to KPKT (2009), an ordinary net mesh or stainless mesh

can be used at roof drain and gutter. It is suggested by the manual that installation of the

leaf guarder shall adopt a net or screen mesh of 2 to 10-mm is satisfactory, as shown in

Figure 3.6.1.

Figure 3.6.1: Net of gutter(KPKT, 2009)

3.6.1 Types of Leaf Guarder

There are numerous types of leaf guarders available in the local and overseas market. It is

noted that installation of gutter leaf guarder depends on the size of the gutter.Figure3.6.2

shows someexamples of leaf guarderavailable in local and overseas.

Some of the available materials include powder coated steel, Stainless steel, black

rubberized vinyl, industrial strength nylon, High Density Industrial Strength Polyethylene

(HDPE) and aluminum.

Figure 3.6.2: Types of leaf guarder

3.6.2 Advantages and Disadvantages

There are pros and cons for installing leaf guarders to the rain gutters. Installation of leaf

guarder ensures that the collected rainwater will be free from large particles that are

undesirable. As mentioned previously in the introduction, leaf guarders do eliminates the

risk of clogging of large particles for the subsequent components of the rainwater

harvesting system. Therefore, safeguard the quality of the rainwater from debris at the

first stage of the filtration.

However, installing a leaf guarder requires periodically checking and

maintenance. As large particles and debris accumulated to a certain amount that may clog

the leaf guarder if no maintenance. There are numerous types of leaf guarders available in

the market currently, as shown in Figure 3.6.2, which can overcome these disadvantages.

3.7 First Flush System

Rainwater is one of the purest forms of water and is initially clean to be used. When it

rains, rainwater wash down from roof where contamination occurs. The rainwater may

collect certain amount of undesirable matters from the roof such as fecal matter, dead

animal bodies, chemical residues, sediments, bacteria and etc. This rainwater is also

known as first flush water. Therefore, a first flush diverter is necessary to carry out this

first stage filtration. When the first flush water is removed, bacterial activities from fecal

bacteria and other water borne bacteria would be greatly reduced. Therefore rainwater

harvested in the system is generally cleaner and safer to be used.

3.7.1 Typical Design of First Flush Diverter

Figure 3.7.1 illustrates the typical type of T-junction type first flush diverter for normal

residential buildings. The device is usually placed at in between the installation of gutter

system and storage tank. When rainwater is flushed down from the roof, gutters and

downpipes will divert the contaminated first flush water into the first flush diverter as

shown in the illustration. Once the contaminated first flush water has filled up the device,

the following rainwater will flow into the storage tank through pipe system. Typically a

floating ball valve or sealing ball is installed in the device to prevent the contaminated

first flush water from washing back out and flow into the storage tank. A small opening

valve is provided at the end of the device to ensure that the device can slowly drain the

water out and reset to accommodate the first flush of next rainfall. Installation of a screw

cap before the opening of the device allows periodically cleaning of debris.

Figure 3.7.1: Typical First Flush Diverter(Rain Harvest, 2013)

3.7.2 Types of First Flush Diverters

There are various types of first flush diverters with different installation methods and

locations available in the market. They are available in diverse material and sizes depend

on the requirement of the users and the connections to the whole rainwater harvesting

system. Basically, common examples of the device are downpipe first flush diverters,

post/wall stand water diverters, commercial diverters and in-ground diverters.

Figure3.7.2 shows a brief introduction of the different types of first flush diverters and

their mechanisms (Rain Harvest, 2013).

Types of first flush Mechanism

Downpipe first flush diverters

Post/wall stand water diverters

In ground diverter

Figure 3.7.2: Types of first flush diverters and the mechanisms (Rain Harvest, 2013)

3.7.3 NAHRIM’s First Flush Diverter

Fitting an appropriately sized First Flush Water Diverter is critical to achieve good

quality water. Water Diverters improve water quality, reduce tank maintenance and

protect pumps by preventing the first flush of water, which may contain contaminants

from the roof, from entering the tank. When it rains, instead of flowing to the rainwater

storage tank directly, the first flush of water from the roof that may contain amounts of

bacteria from decomposed insects, bird and animal droppings and concentrated tannic

acid, is diverted into the chamber of the first flush water diverter. Figure 3.7.3a shows

the illustration of NAHRIM’s First Flush Diverter and Figure 3.7.3b shows the detailed

components of the diverter.

Figure 3.7.3a:Illustration of NAHRIM’s First Flush Diverter

This water diverter utilises a dependable ball and seat system, which is a simple

automatic system. As the water level rises in the chamber, the ball floats. Once the

chamber is full, the ball rests on a seat inside the chamber preventing any further water

entering the chamber. The subsequent of fresh water is then channeled into the rainwater

storage tank through an insect screen. A slow release valve ensures the chamber empties

itself after rain and resets automatically.

Figure 3.7.3b: The detailed components of the diverter

3.7.4 Volume of First Flush Diverter

Users could also design the volumes of their first flush diverters. A minimum design

criterion is that the first flush device should divert the first 0.5-mm (or, 1.0-mm) of the

rainfall (first flush depth). To calculate the volume of rainwater needed to be diverted,

multiplies the length and width of the roof by the first flush depth.

Required volume of diverted water (m3) = roof length (m) * roof width (m) * first flush depth (m)

Eq. 3.7a

For example, a house with 10-m long by 5-m wide would need to divert at least

0.025-m3 (or, 0.05-m3 if 1.0-mm of first flush depth is used) of first flush. This is the

amount of first flushthat a simple-pipe first flush device would have to divert. By

dividing the required volume of first flushwith the cross sectional area of the pipe (πr2),

the necessary pipe length for the simple-pipe first flush device can be calculated from the

following equation:

Pipe length (m) = Required volume of diverted water (m3) / πr2

Eq. 3.7b

A first flush downpipe of 200 mm diameter (100 mm radius) would need to be at

least 0.8-m (1.6-m if1.0-mm of first flush depthis used) long.

3.7.5 The Malaysian’s Condition

3.7.5.1 KPKT

According to KPKT (2009), the dimension of the first flush device to be adopted is with

minimum diameter of 100-mm and the layout of design is shown in Figure 3.7.4 below.

Figures 3.7.5a and 3.7.5b show the types of first flush diverter suggested by KPKT.

Figure 3.7.4: Device to separate first flush rainwater(KPKT, 2009)

Figure 3.7.5a: Standpipe First Flush Diverter(KPKT, 2009)

Figure 3.7.5b: Standpipe with Ball Valve(KPKT, 2009)

3.7.5.2 DID

The Department of Irrigation and Drainage Malaysia (DID, 2011)gives the requirements

for designing a first flush system in Tables 3.7.1 and 3.7.2.

Table 3.7.1: Guidelines for residential first flush quantities (DID, 2011)

Rooftops of 100m2 or smaller 25-50 liters

Rooftops of 100m2 or larger 50 liters per 100m2

Table 3.7.2: Guidelines for surface catchments or for very large rooftops(DID, 2011)

Rooftops or surface catchments of 4,356m2 or larger 2,500 liters

3.7.5.3 SIRIM Berhad

InSIRIM (2013), it is stated that the first flush system installed in the buildings shall be

able to cater a volume equivalent to 0.5mm of rainfall before the consequent rainwater

entering the storage tank. Table 3.7.3 shows the first flush requirement according to the

roof areas of buildings.

Table 3.7.3: First flush requirement according to roof area(SIRIM, 2013)

Roof area (m2) First flush volume(m3)

<100100 to 4356

>4356

0.025 to 0.050.05 to 2.5

2.5

NOTE. Adopt first flush of 5m3 if surface contains excessive soil, dust or debris.

3.7.5.4 NAHRIM

NAHRIM (2013)stated that the first 1-mm of rainwater from the rooftop is normally

contaminated with undesired particles. All the rainwater harvesting systems installed by

NAHRIM followed this design criterion to ensure good quality of rainwater.

3.7.6 Advantages

It is noted that the first flush diverter is operating automatically, which is simple and user

friendly. Most of the first flush diverter is simple and easy to be installed. It does not

require manual intervention and is almost maintenance free.

The device could safeguard the quality of the rainwater through prevention of

undesirable matters and pollutants from entering the storage tank at the first stage of

rainwater harvesting. Therefore, the rainwater collected could be directly used for non-

potable purpose without further complicated treatments. Besides, without accumulation

of the undesirable contaminants allows protection to the subsequent system. Also, no

power is required to operate a first flush diverter. The technology is a low-tech and low-

cost demand to improve the rainwater quality in ease.

RAIN WATER COLLECTION & HARVESRING

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